Follow Me: ArcPy(2)

import arcpy.mapping as mapping
mxd = mapping.MapDocument("CURRENT")

小笨认为第一行import需要一些额外的解释。首先，请记牢：如果import在最前面，import后面跟着的的只能是模块！（考试必背）

import arcpy.mapping
mxd = arcpy.mapping.MapDocument("CURRENT")

from arcpy import mapping
mxd = mapping.MapDocument("CURRENT")

• (ELF only) Using the DT_RPATH dynamic section attribute of the binary if present and DT_RUNPATH attribute does not exist.  Use of DT_RPATH is deprecated.
• Using the environment variable LD_LIBRARY_PATH.  Except  if  the executable is a set-user-ID/set-group-ID binary, in which case it is ignored.
• (ELF only) Using the DT_RUNPATH dynamic section attribute of the  binary if present.
• From the cache file /etc/ld.so.cache which contains a compiled list of candidate libraries previously found in the augmented library  path.  If, however, the binary was linked with -z node-flib linker option, libraries in the default library paths are skipped.
• In the default path /lib, and then /usr/lib. If the binary was linked with -z nodeflib linker option, this step is skipped.

#include <iostream>
using namespace std;

像这样，只有iostream这个头文件中定义在std这个名空间之内的类型，函数，变量，才会被引入当前代码文件。如果木有第二行的声明，引入定义在ostream中的标准输出流实例cout，就只能使用包含名空间的全名：

std::cout<<"hello!"

其中的“::“就是C++作用域操作符。

import java.util.ArrayList;

有上面的import撑腰，在代码中可以直接引用ArrayList:

ArrayList<String> list = new ArrayList<String>

和C++一样，如果不显式引入ArrayList，定义一个ArrayList就必须使用全名：

java.util.ArrayList<String> list = new java.util.ArrayList<String>

是不是多敲了很多字符啊？所以Java的import，其实就是个“语法糖“。小笨比较懒，木有深入过Java编译器，但据小笨猜测，这个全名的替换过程也是在预处理阶段完成的。在运行时木有作用。 那Java里库是如何被加载的呢？当然是运行时呗！大家都知道Java程序的byte code是由虚拟机加载执行的。在虚拟机执行程序之前，class loader会得到一个需要加载的包的名单，然后根据预先指定的包路径按图索骥。如果啥都木有发现，jvm就会抛给你一个经典的运行时异常：ClassNotFoundException。。。这个过程和import没有半毛钱的关系！

package Barfoo;
public class Foobar{
......
}

这就意味着Barfoo中的类Foobar可以被任何其他类import（引用），无论是Barfoo包内还是包外。反之，如果我们忽略访问修饰符：

package Barfoo;
class Foobar{
......
}

• Bootstrap class loader　// 负责加载　JAVA_HOME/jre/lib　内的包包
• Extensions class loader　// 负责加载　JAVA_HOME/jre/lib/ext　或者　java.ext.dirs　内指定的包包
• System class loader/Application class loader　// 负责加载 CLASSPATH (java.class.path)　上的包包

T::bar * p;

上面的代码歧义不？

typename T::bar * p;

1. import <module/package, full qualified name>
2. from <module/package, full qualified name> import whatever_the_shit_might_be
3. import <module/package, full qualified name> as whatever_local_variable_u_want_to_call_this_module
4. from <module/package, full qualified name> import whatever_the_shit_might_be as whatever_local_variable_u_want_to_call_this_whatever_shit
5. from  <module/package, full qualified name> import *

import Foo
import Foo.Foobar
from Foo import Foobar
import Foo.Foobar as foobar
from Foo import Foobar as foobar
from Foo.Foobar import SB
from Foo.Foobar import SB as sb
from Foo.Foobar import DSB
from Foo.Foobar import DSB as dsb
from Foo.Foobar import SB, DSB
from Foo.Foobar import SB as sb, DSB as dsb
from Foo.Foobar import *

以下是非法的引用：

import Foo.Foobar.SB
import Foo.Foobar.DSB

为啥呢？老师叫你背过了嘛：如果import在最前面，import后面跟着的的只能是模块！SB不是模块，只能通过from…import…来引入。

import arcpy.mapping
mxd = arcpy.mapping.MapDocument("CURRENT")

from arcpy import mapping
mxd = arcpy.mapping.MapDocument("CURRENT")

import arcpy.mapping
mapping=arcpy.mapping
mxd = mapping.MapDocument("CURRENT")

#module:Bar.py

A=10
B=[1,10]

#script1.py
from Bar import A, B


A=100
B[0]=1000

import sys

for name in sys.builtin_module_names:
    print name

转载自：https://blog.csdn.net/micropentium6/article/details/42360983